The present invention relates to a bipolar type ion exchange membrane electrolytic cell which is suitably useful for the production of e.g. an aqueous alkali metal hydroxide solution.
Heretofore, as an ion exchange membrane electrolytic cell to be used for e.g. production of an aqueous alkali metal hydroxide solution, a filter press type electrolytic cell has been used in many cases. This is one wherein a number of ion exchange membranes and compartment frame units each comprising an anode compartment frame and a cathode compartment frame, are alternately arranged and clamped from both sides by e.g. a hydraulic press. Types of electrolytic cells are generally classified into a monopolar type electrolytic cell (monopolar cell) of a parallel connection type and a bipolar type electrolytic cell (bipolar cell) of a series connection type, which are distinguishable by the difference in electrical connection.
As shown in FIGS. 1 and 2, in a compartment frame unit (general term for an anode compartment frame and a cathode compartment frame) for a bipolar type electrolytic cell, an anode compartment 15 and a cathode compartment 25 are arranged back to back, and an anode compartment frame 10 constituting the anode compartment 15, comprises an anode plate 30 and an anode back plate 40 arranged in substantially parallel with the anode plate with a spacing therefrom. As such an anode plate, it is common to employ a meshed or porous plate. For example, a conductive meshed plate of e.g. titanium, zirconium or tantalum is used as a substrate, and an oxide of a noble metal such as titanium oxide, ruthenium oxide or iridium oxide, is coated thereon.
Between the anode plate 30 and the anode back plate 40, corrosion resistant conductive anode supporting members (called also as ribs) 50a made of e.g. titanium or a titanium alloy, are arranged with a prescribed spacing from one another to electrically connect the two and to maintain the spacing therebetween. Each anode supporting member 50a may, for example, be made of a plate member and provided with a plurality of through-holes (not shown) so that an electrolyte can flow in the left and right directions in FIGS. 1 and 2.
The construction of the cathode compartment frame 20 for providing a cathode compartment 25 is the same as that of the anode compartment frame 10. Namely, it comprises a meshed or porous cathode plate 60, a cathode back plate 70 and cathode supporting members 80a. 
Similarly, between the cathode plate 60 and the cathode back plate 70, corrosion resistant conductive cathode supporting members 80a made of e.g. iron, nickel or a nickel alloy, are arranged with a prescribed spacing from one another to electrically connect the two and to maintain the spacing therebetween, as shown e.g. in FIG. 1.
The anode back plate 40 and the cathode back plate 70 are integrally connected to form a partition wall 9. Between the anode back plate 40 and the cathode back plate 70 constituting the partition wall 9, a conductive interlayer member such as a cladding material (not shown) may be inserted in order to increase the electrical conductivity. A peripheral edge portion of each of the anode back plate 40 and the cathode back plate 70 constituting the partition wall, is bent and fixed to a hollow body 7 by e.g. welding. Reference numeral 11 indicates an ion exchange membrane, and numeral 12 a gasket. The cathode plate is preferably made of an alkali resistant material, such as a substrate made of e.g. a conductive meshed plate of e.g. nickel or stainless steel, coated with a cathode active material such as Raney nickel or a platinum series.
In a case where such a bipolar cell is used for electrolysis of an alkali metal halide such as sodium chloride to produce an alkali metal hydroxide, an almost saturated sodium chloride aqueous solution is supplied as an anolyte to an anode compartment from an anolyte inlet 3 which is usually provided at a lower portion of the anode compartment. In the anode compartment, chlorine gas is generated on the anode plate by electrolysis, and it will be discharged, together with the aqueous sodium chloride solution as the electrolyte, out of the anode compartment frame from an anolyte outlet 4 which is provided usually at an upper portion of the anode compartment.
On the other hand, in a cathode compartment, water or a dilute sodium hydroxide aqueous solution is supplied as a catholyte to the cathode compartment from a catholyte inlet 5 which is provided usually at a lower portion of the cathode compartment. In the cathode compartment, hydrogen gas and sodium hydroxide are formed and discharged out of the cathode compartment from a catholyte outlet 6 which is provided at an upper portion of the cathode compartment.
The role of an ion exchange membrane used for this sodium chloride electrolysis, is to let sodium ions pass from the anode compartment side to the cathode compartment side and to shut off movement of hydroxyl ions generated on the cathode side to the anode compartment side.
Usually the anode plate 30 is fixed to e.g. anode supporting members 50a in the anode compartment by e.g. welding. Likewise, the cathode plate 60 is also fixed to e.g. cathode supporting members 80a in the cathode compartment by e.g. welding, and the anode plate 30 and the cathode plate 60 are clamped with an ion exchange membrane interposed via gaskets 12 so that they maintain a prescribed distance. In general, the distance between the anode plate and the cathode plate (the anode-cathode distance) is a factor giving a substantial influence over the electrolysis voltage of the electrolytic cell. As a matter of course, the shorter the anode-cathode distance, the lower the electrolysis voltage, so that the electric power can be saved. On the other hand, if the anode and the cathode are too close to each other, the electrode plates are likely to contact with the membrane, since the membrane itself is flexible, and its position in the electrolyte is not completely fixed. In such a case, as numerous fine irregularities or projections are present on the surface of the electrode plates, if the membrane moves in frictional contact with the electrode plate surface in such a state that these irregularities or projections are forcibly pressed against the membrane, the membrane is likely to be forcibly cut.
If a substantial portion of the membrane is thus damaged, normal operation of the electrolytic cell tends to be finally impossible. Accordingly, heretofore, the operation is obliged to be carried out on a safe side by increasing the anode-cathode distance to such an extent where there will be no possibility of damaging the membrane, even if the electrolysis voltage is sacrificed to some extent.
Some attempts have been proposed in the past not to give a damage to an ion exchange membrane even if the membrane is disposed as close as possible to an anode plate or a cathode plate having such fine irregularities or projections. For example, JP-A-57-108278 discloses a technique wherein a number of conductive spring members are provided between an electrode plate and a partition plate on the anode side and/or the cathode side to make the electrode plate movable. Further, JP-A-1-55392 discloses a technique wherein a partition plate and an electrode plate are electrically connected by a clamp spring mechanism, and at the same time, the electrode plate is made movable by the resilience of the clamp spring mechanism.
These are techniques whereby even if the electrode plate and a membrane are in contact with each other, the pressing pressure can be reduced, but each employs a movable mechanism by springs, whereby there has been a problem such that (1) the electrical resistance at the spring member portions increases, or (2) the production costs tend to increase because of the. complexity in the structure of the spring mechanism. (3) A more serious problem is that since a movable mechanism whereby the spacing between the electrode and the partition wall is maintained solely by the spring members having resiliency, even if it is possible to make the electrode plate movable, it is necessarily impossible from its mechanism to maintain the anode-cathode distance which must be uniformly maintained over the entire electrolytic surface. Therefore, even if it is possible on appearance to reduce the anode-cathode distance by the movable mechanism, in reality, it is impossible to maintain the uniformity of the anode-cathode distance during a steady operation, and from the overall viewpoint, it has been impossible to effectively reduce the electrolysis voltage.
It is an object of the present invention to solve such problems and to provide a bipolar type ion exchange electrolytic cell which is capable of reducing the electrolysis voltage substantially by minimizing the anode-cathode distance by a simple and inexpensive movable mechanism having a low electrical resistance. Further, it is an object of the present invention to provide a bipolar type ion exchange electrolytic cell whereby even if the spacing between the electrode plate and the ion exchange membrane is made to be from 0.1 to 1.0 mm, there will be no danger of damage to the membrane.
Firstly, the present invention provides the following invention.
A bipolar type ion exchange membrane electrolytic cell comprising an anode compartment frame which comprises an anode plate and an anode back plate arranged in substantially parallel with each other with a spacing, conductive anode supporting members arranged with a prescribed spacing from one another between the anode plate and the anode back plate, and a cathode compartment frame which comprises a cathode plate and a cathode back plate arranged in substantially parallel with each other with a spacing, and conductive cathode supporting members arranged with a prescribed spacing from one another between the cathode plate and the cathode back plate, so that the respective back plates are connected back to back to form a compartment frame unit, a plurality of such compartment frame units being arranged with a cation exchange membrane interposed, wherein
(a) at least the cathode supporting members comprise electric current supply rib base portions fixed to the cathode back plate and standing up towards the cathode plate, and a flexible member supported by the adjacent electric current supply rib base portions and extending to reach the cathode plate,
(b) the flexible member and the cathode plate are electrically connected to each other via a connecting portion of the flexible member, and
(c) electric current supply from the cathode plate to the electric current supply rib base portions is carried out through the connecting portion, and the cathode plate is movably supported by the function of the flexible member.
Secondly, the present invention provides the following invention.
A bipolar type ion exchange membrane electrolytic cell comprising an anode compartment frame which comprises an anode plate and an anode back plate arranged in substantially parallel with each other with a spacing, conductive anode supporting members arranged with a prescribed spacing from one another between the anode plate and the anode back plate, and a cathode compartment frame which comprises a cathode plate and a cathode back plate arranged in substantially parallel with each other with a spacing, and conductive cathode supporting members arranged with a prescribed spacing from one another between the cathode plate and the cathode back plate, so that the respective back plates are connected back to back to form a compartment frame unit, a plurality of such compartment frame units being arranged with a cation exchange membrane interposed, wherein
(a) at least the anode supporting members comprise electric current supply rib base portions fixed to the anode back plate and standing up towards the anode plate, and a flexible member supported by the adjacent electric current supply rib base portions and extending to reach the anode plate,
(b) the flexible member and the anode plate are electrically connected to each other via a connecting portion of the flexible member, and
(c) electric current supply from the electric current supply rib base portions to the anode is carried out through the connecting portion, and the anode plate is movably supported by the function of the flexible member.